1. Field of the Invention
The present invention relates to bipolar-based active pixel sensor cells and, more particularly, to a bipolar-based active pixel sensor cell with a metal contact and increased capacitive coupling to the base region of the cell.
2. Description of the Related Art
Charge-coupled devices (CCDs) have been the mainstay of conventional imaging circuits for converting a pixel of light energy into an electrical signal that represents the intensity of the light energy. In general, CCDs utilize a photogate to convert the light energy into an electrical charge, and a series of electrodes to transfer the charge collected at the photogate to an output sense node.
Although CCDs have many strengths, which include a high sensitivity and fill-factor, CCDs also suffer from a number of weaknesses. Most notable among these weaknesses, which include limited readout rates and dynamic range limitations, is the difficulty in integrating CCDs with CMOS-based microprocessors.
To overcome the limitations of CCD-based imaging circuits, more recent imaging circuits use bipolar-based active pixel sensor cells to convert a pixel of light energy into an electrical signal. FIG. 1 shows an example of a conventional bipolar-based active pixel sensor cell 10 with a capacitively coupled base region.
As shown in FIG. 1, cell 10 includes an n-well 14, which functions as a collector, formed in a p-type substrate 12; a p-type region 16, which functions as a base, formed in collector region 14; and an n+region 18, which functions as an emitter, formed in base region 16.
In addition, cell 10 also includes a field oxide region FOX formed in collector region 14 adjoining base region 16, a layer of gate oxide 20 formed on base region 16 and an outer portion of emitter region 18, and an n+ polysilicon (poly) line 22 formed on the field oxide region FOX and a portion of the layer of gate oxide 20 formed over base region 16.
Poly line 22 is conventionally doped n+ rather than p+ because additional masking steps would be required to dope poly line 22 with a p-type material. In addition, a p-type material heavily implanted into poly line 22 can easily diffuse into and damage the layer of gate oxide 20.
As further shown in FIG. 1, cell 10 additionally includes a layer of oxide 24 formed on polysilicon line 22 and a portion of the layer of gate oxide 20, and a metal contact 26 formed on a center area of emitter region 18 adjoining the layer of oxide 24.
Operation of active pixel sensor cell 10 is performed in two steps: an image integration step, where the light energy is collected and converted into an electrical signal; and a signal readout step, where the signal is read out.
At the beginning of the image integration step, the base-emitter junction is reverse biased by applying a fixed voltage to poly line 22. The voltage applied to poly line 22 is capacitively coupled to base region 16 by a coupling capacitor that utilizes poly line 22 as the top plate, gate oxide layer 20 as the dielectric, and base region 16 as the bottom plate. In addition, the collector-base junction is also reverse-biased by applying a fixed voltage, such as Vcc, to collector region 14.
During the image integration step, light energy, in the form of photons, strikes cell 10, thereby creating a number of electron-hole pairs. Under these conditions, the holes formed in base region 16 remain in base region 16, while the holes formed in collector region 14 and emitter region 18 diffuse to base region 16, where each additional hole in base region 16 increases the charge on base region 16.
At the end of the integration step, cell 10 is read out by pulsing poly line 22 with a positive voltage which, in turn, increases the voltage on base region 16. The increased voltage on base region 16, in combination with the increased charge due to the collected holes, forward-biases the base-emitter junction causing an amplified current to flow from emitter region 18 into metal contact 26 that is proportional to the number of collected holes.
One problem with cell 10, however, is that the capacitance of the coupling capacitor is relatively low due to the limited area that is available to form the capacitor. As a result, the fixed and pulsed voltages present on base region 16 are substantially less than the fixed and pulsed voltages applied to poly line 22, thereby limiting the dynamic range of cell 10.
Another problem with cell 10 is that the voltage placed on poly line 22 may cause the surface of base region 16 to become inverted, thereby effectively increasing the size of emitter region 18. By increasing the effective size of emitter region 18, the leakage current associated with the p-n junction also increases, thereby increasing the noise level.
Thus, there is a need for a bipolar-based active pixel sensor cell that increases the dynamic range of the cell, and reduces the noise associated with the cell.